Commercially available epoxy resins in combination with commercially available hardeners produce materials with a broad application as coatings for corrosion protection, components of composite materials and as moulding plastics. In addition to the basic components of epoxy resin and hardener the starting materials may contain dyes, pigments, fillers, reactive and non-reactive diluents, volatile solvents, stabilizing agents and additives.
Epoxy resins usually contain more than one 1,2-epoxy group per mole and may be based on saturated, unsaturated, aromatic, aliphatic, cycloaliphatic or heterocyclic structures.
Hardeners are usually chosen from the following groups of chemical compounds: aromatic, aliphatic, cycloaliphatic or heterocyclic amines, amine adducts, polyamides, polyamido amides, Mannich bases, ketimines or carboxylic acid derivatives. Mercaptan compounds can also be used as active compounds within the hardener.
Fillers include titanium dioxide, silica, diverse silicates, minerals or carbon black
Stabilisers include antioxidants, radical scavengers or UV-absorbers.
Additives include plasticizers, catalysts for the curing reaction, rheology modifying additives or surfactants.
Reactive diluents are often epoxy compounds of considerably lower viscosity than the epoxy resins.
It is known that the colour stability of materials which are made from commercially available epoxy resins and hardeners is often poor because the hardeners or combination of hardener, resin and additives, have a strong tendency to yellowing, also after curing. A known method of reducing yellowing is to use amine based hardeners with aliphatic or cycloaliphatic structures, because in the presence of light the yellowing of cycloaliphatic amines is significantly less than that of aromatic amines.
The disadvantage of using aliphatic or cycloaliphatic amines as hardeners or curing agents is that the abrasion and scratch resistance of the resulting materials is often poorer than for aromatic amines.
It is also known that the abrasion, scratch and chemical resistance of cured epoxies can be significantly improved by the use of fillers such as silica (U.S. Pat. No. 3,794,609). The disadvantage however is that the transparency of the hardened epoxy is considerably reduced, which is perceived as detrimental, particularly when the material is intended for use as coating.
A known method of producing cured epoxies with high colour stability, abrasion, scratch and chemical resistance, and acceptable transparency can therefore be to use hardeners based on aliphatic or cycloaliphatic amines with low yellowing tendency together with silica based nanoparticles as an additive. One example of silica based nanoparticles is the Aerosil® products of Degussa AG, Germany. From EP 0774443 A1 it is known that nanodisperse titanium dioxide is suitable for improving the colour stability of, amongst others, polymer based formulations.
An alternative method for the preparation of coatings with good abrasion, scratch and chemical resistance together with acceptable transparency is based on organic, polymer forming components and inorganic, particle containing or particle forming components where the particle size is between 1 and 150 nm. The coating is usually cured by applying the mixture of organic and inorganic components to a surface and drying with the aid of heat and/or UV-VIS radiation. Such coating forming mixtures may contain epoxy resins or compounds with epoxy groups. A large number of patents and publications exist which describe the preparation of such organic-inorganic hybrid materials and possible applications: JP 09132637, U.S. Pat. Nos. 5,618,860, 5,804,616, WO 9832792 EP 496552, KR 2000059589, JP 2001288401 and Milena Spirkova et. al. ”Hybrid Organic-Inorganic Epoxide-Based Coatings Prepared by Sol-Gel Process”, Proceedings of 6th Nürnberg Congress on Creative Advances in Coatings Technology”, paper 12 (2001).
Thermosetting plastics such as epoxy resins can also be modified with nanodisperse inorganic particles for applications other than coatings. DE 198 60 691 A1 describes a magnetic paste which contains nanocrystals. WO 9631572 A1 describes polymerisable, nanoparticle containing formulations which, amongst others, are based on acrylic or epoxy resins and which can be used for building up or joining of optoelectronic elements. WO 0130304 A1 describes materials which are based on organic thermosets and inorganic nanoparticle containing or nanoparticle forming components. The materials are used as dental replacement materials. In addition a number of scientific publications describe the modification of thermosetting plastics such as epoxy resins with nanoparticle containing or nanoparticle forming mixtures (e.g. Soo-Jin Park et. al. ”Surface Modification of Montmorillonite on Surface Acid-Base Characteristics of Clay and Thermal Stability of Epoxy/Clay Nanocomposites”, Journal of Colloid and Interface Science 251, 160-165 (2002)).
Also the preparation of nitrogen containing, basic hardeners or curing agents with hydrolisable silane compounds for thermosets such as epoxy resins is known. U.S. Pat. No. 4,988,778 describes hardeners which are prepared by partial alcoholysis/aminolysis of γ-aminopropyltrimethoxy silane with diisopropyl amine, but without the addition of water. JP 04366159 describes a product which is made by the reaction of γ-glycidopropyltrimethoxy silane with water and small quantities of an amidine compound 1,8-diazabicyclo[5.4.0]undeken-7 and which is used as a subcomponent for curing of an epoxy containing thermosetting mixture. The basic amidine compound constitutes however less than 62 ppm by weight of the silane/water mixture thus the product itself must be considered as unsuitable as a hardener for epoxy resins.
It is also shown in PCT/NO2001/00287 that an existing organic lacquer or gelcoat can be modified with suitable nanoparticle mixtures to, amongst other things, give improved scratch resistance. Here a stable nanoparticle containing sol is prepared which is added to the existing organic lacquer or gelcoat as required.
However, we believe, that the use of sol-gel mixtures, which are prepared by controlled hydrolysis/condensation of γ-aminopropyltrialkoxy silane or other nitrogen containing silanes and/or mercaptosilanes as hardener for epoxy resins has not been documented.
The sol-gel process is a simple way of preparing nanoparticle based mixtures. The sol-gel process is based on a controlled hydrolysis/condensation of e.g. silane alkoxides. The process is described in PCT/NO2001/00287 and yields gels which relatively easily can be mixed into polymeric and/or polymerisable organic formulations.
One example is sols which are prepared by controlled hydrolysis/condensation of γ-aminopropyltrialkoxy silane. The sol-gel process is in this case particularly simple because an external catalyst is not required and because the process can be performed at room temperature or with gentle heating.
Known hardeners for epoxy resins are based on amines such as 4,4′-diaminodiphenylmethane (I) or meta-xylylenediamine (II):
Amines react with epoxy resins by crosslinking with two or more polymer chains in the epoxy resins. This leads to solidification (hardening) of the two component system which consists of amine based hardener and epoxy resin and the formation of a relatively abrasion resistant material. The disadvantage of amine based hardeners is that the hardened material yellows relatively quickly due to, amongst other things, oxidative degradation of components in the amine based hardener.
In addition the abrasion/scratch resistance of the hardened material is often too poor for the material to be used for demanding applications.